Demo
 - web(接入層)
  - src
   - main
    - java
    - resources
     - application.properties // 1 
   - test
  - pom.xml
 - common(公共層)
  - src
   - main
    - java
    - resources
     - application-dev.properties // 2
 - pom.xml(父Module pom)

A /config subdirectory of the current directory(file:./config/)
The current directory(file:./)
A classpath /config package(classpath:/config/)
The classpath root(classpath:/)

src
 - main
  - resources
   - config
    - application.properties // 3 (k1=v1, k2=v2)
   - application.properties // 4  (k1=v3, k4=v4)

// org.springframework.boot.SpringApplication#run(java.lang.String...)
public ConfigurableApplicationContext run(String... args) {
 // ...(省略)
 listeners.starting();
 try {
  ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
  // 創(chuàng)建ConfigurableEnvironment
  ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
  // ...(省略)
}

// org.springframework.boot.SpringApplication#run(java.lang.String...)
private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
  ApplicationArguments applicationArguments) {
 // Create and configure the environment
 ConfigurableEnvironment environment = getOrCreateEnvironment();
 configureEnvironment(environment, applicationArguments.getSourceArgs());
 ConfigurationPropertySources.attach(environment);
 // 發(fā)布ApplicationEnvironmentPreparedEvent事件
 listeners.environmentPrepared(environment);
 // ...(省略)
}

// org.springframework.boot.SpringApplicationRunListeners#environmentPrepared
public void environmentPrepared(ConfigurableEnvironment environment) {
 for (SpringApplicationRunListener listener : this.listeners) {
  listener.environmentPrepared(environment);
 }
}

// org.springframework.boot.context.event.EventPublishingRunListener#environmentPrepared
public void environmentPrepared(ConfigurableEnvironment environment) {
    // 發(fā)布ApplicationEnvironmentPreparedEvent事件
 this.initialMulticaster
   .multicastEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args, environment));
}

// org.springframework.boot.context.config.ConfigFileApplicationListener
private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
 // 利用Spring SPI機制加載EnvironmentPostProcessor
 List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
 postProcessors.add(this);
 AnnotationAwareOrderComparator.sort(postProcessors);
 for (EnvironmentPostProcessor postProcessor : postProcessors) {
  // 回調
  postProcessor.postProcessEnvironment(event.getEnvironment(), event.getSpringApplication());
 }
}

// org.springframework.boot.context.config.ConfigFileApplicationListener
public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
 addPropertySources(environment, application.getResourceLoader());
}

protected void addPropertySources(ConfigurableEnvironment environment, ResourceLoader resourceLoader) {
    // 添加`RandomValuePropertySource`到Environment
 RandomValuePropertySource.addToEnvironment(environment);
 // load()方法是重點;
 new Loader(environment, resourceLoader).load();
}

// org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#load()

public void load() {
 this.profiles = new LinkedList<>();
 this.processedProfiles = new LinkedList<>();
 this.activatedProfiles = false;
 this.loaded = new LinkedHashMap<>();
 // 以上四個變量默認狀態(tài)為空集合或false，用于在下邊迭代的過程中收集數(shù)據(jù)
 // 初始化profiles集合，如果存在active的profile，會將activatedProfiles變量設置為true
 initializeProfiles();
 while (!this.profiles.isEmpty()) {
  Profile profile = this.profiles.poll();
  if (profile != null && !profile.isDefaultProfile()) {
   addProfileToEnvironment(profile.getName());
  }
  load(profile, this::getPositiveProfileFilter, addToLoaded(MutablePropertySources::addLast, false));
  this.processedProfiles.add(profile);
 }
 resetEnvironmentProfiles(this.processedProfiles);
 load(null, this::getNegativeProfileFilter, addToLoaded(MutablePropertySources::addFirst, true));
 addLoadedPropertySources();
}

private void initializeProfiles() {
 // The default profile for these purposes is represented as null. We add it
 // first so that it is processed first and has lowest priority.
 this.profiles.add(null);
 Set<Profile> activatedViaProperty = getProfilesActivatedViaProperty();
 this.profiles.addAll(getOtherActiveProfiles(activatedViaProperty));
 // Any pre-existing active profiles set via property sources (e.g.
 // System properties) take precedence over those added in config files.
 addActiveProfiles(activatedViaProperty);
 if (this.profiles.size() == 1) { // only has null profile
  for (String defaultProfileName : this.environment.getDefaultProfiles()) {
      // 加載名為`default`的profile
   Profile defaultProfile = new Profile(defaultProfileName, true);
   this.profiles.add(defaultProfile);
  }
 }
}

private void load(Profile profile, DocumentFilterFactory filterFactory, DocumentConsumer consumer) {
 getSearchLocations().forEach((location) -> {
  boolean isFolder = location.endsWith("/");
  Set<String> names = isFolder ? getSearchNames() : NO_SEARCH_NAMES;
  names.forEach((name) -> load(location, name, profile, filterFactory, consumer));
 });
}

private Set<String> getSearchLocations() {
 // 若通過 spring.config.location 指定配置文件目錄，則到指定路徑查找，不再走默認的搜索路徑和額外添加的路徑，可以指定多個，以逗號進行分隔
 // CONFIG_LOCATION_PROPERTY = spring.config.location
 if (this.environment.containsProperty(CONFIG_LOCATION_PROPERTY)) {
  return getSearchLocations(CONFIG_LOCATION_PROPERTY);
 }
 
 // 除了默認路徑，還可以通過 spring.config.additional-location 指定額外的搜索路徑
 // CONFIG_ADDITIONAL_LOCATION_PROPERTY = spring.config.additional-location
 Set<String> locations = getSearchLocations(CONFIG_ADDITIONAL_LOCATION_PROPERTY);
 
 // 默認搜索路徑
 // DEFAULT_SEARCH_LOCATIONS = classpath:/,classpath:/config/,file:./,file:./config/
 locations.addAll(
   asResolvedSet(ConfigFileApplicationListener.this.searchLocations, DEFAULT_SEARCH_LOCATIONS));
 return locations;
}

private Set<String> asResolvedSet(String value, String fallback) {
 List<String> list = Arrays.asList(StringUtils.trimArrayElements(StringUtils.commaDelimitedListToStringArray(
   (value != null) ? this.environment.resolvePlaceholders(value) : fallback)));
 Collections.reverse(list);
 return new LinkedHashSet<>(list);
}

private Set<String> getSearchNames() {
 // 若通過 spring.config.name 指定配置文件名稱，則只會搜索該名稱的配置文件，可以指定多個，以逗號進行分隔
 // CONFIG_NAME_PROPERTY = spring.config.name
 if (this.environment.containsProperty(CONFIG_NAME_PROPERTY)) {
  String property = this.environment.getProperty(CONFIG_NAME_PROPERTY);
  return asResolvedSet(property, null);
 }

 // 默認搜索的配置文件名稱為application
 // DEFAULT_NAMES = application
 return asResolvedSet(ConfigFileApplicationListener.this.names, DEFAULT_NAMES);
}

private void load(String location, String name, Profile profile, DocumentFilterFactory filterFactory, DocumentConsumer consumer) {
 // ...(省略)
 Set<String> processed = new HashSet<>();
 for (PropertySourceLoader loader : this.propertySourceLoaders) {
  for (String fileExtension : loader.getFileExtensions()) {
      // .properties\.xml\.yml\.yaml
   if (processed.add(fileExtension)) {
    loadForFileExtension(loader, location + name, "." + fileExtension, profile, filterFactory,
      consumer);
   }
  }
 }
}

private void loadForFileExtension(PropertySourceLoader loader, String prefix, String fileExtension, Profile profile, DocumentFilterFactory filterFactory, DocumentConsumer consumer) {
 DocumentFilter defaultFilter = filterFactory.getDocumentFilter(null);
 DocumentFilter profileFilter = filterFactory.getDocumentFilter(profile);
 if (profile != null) {
  // Try profile-specific file & profile section in profile file (gh-340)
  // profileSpecificFile = file:./application-dev.properties
  String profileSpecificFile = prefix + "-" + profile + fileExtension;
  // 加載profile對應的配置文件
  load(loader, profileSpecificFile, profile, defaultFilter, consumer);
  load(loader, profileSpecificFile, profile, profileFilter, consumer);
  // Try profile specific sections in files we've already processed
  for (Profile processedProfile : this.processedProfiles) {
   if (processedProfile != null) {
    String previouslyLoaded = prefix + "-" + processedProfile + fileExtension;
    load(loader, previouslyLoaded, profile, profileFilter, consumer);
   }
  }
 }
 // Also try the profile-specific section (if any) of the normal file
 // 加載非profile的配置文件
 load(loader, prefix + fileExtension, profile, profileFilter, consumer);
}

private void load(PropertySourceLoader loader, String location, Profile profile, DocumentFilter filter, DocumentConsumer consumer) {
 try {
  Resource resource = this.resourceLoader.getResource(location);
  // ...(省略)
  String name = "applicationConfig: [" + location + "]";
  List<Document> documents = loadDocuments(loader, name, resource);
  // ...(省略)
  List<Document> loaded = new ArrayList<>();
  for (Document document : documents) {
   if (filter.match(document)) {
    addActiveProfiles(document.getActiveProfiles());
    addIncludedProfiles(document.getIncludeProfiles());
    loaded.add(document);
   }
  }
  Collections.reverse(loaded);
  if (!loaded.isEmpty()) {
   loaded.forEach((document) -> consumer.accept(profile, document));
   // ...(省略)
}

// org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#addLoadedPropertySources
private void addLoadedPropertySources() {
 MutablePropertySources destination = this.environment.getPropertySources();
 List<MutablePropertySources> loaded = new ArrayList<>(this.loaded.values());
 Collections.reverse(loaded);
 String lastAdded = null;
 Set<String> added = new HashSet<>();
 for (MutablePropertySources sources : loaded) {
  for (PropertySource<?> source : sources) {
   if (added.add(source.getName())) {
    addLoadedPropertySource(destination, lastAdded, source);
    lastAdded = source.getName();
   }
  }
 }
}